1. Field of the Invention
This invention relates, in general, to the field of isolation amplifiers which utilize magnetic (i.e. transformer) coupling. More specifically, however, the invention relates to an isolation amplifier with a modulator for reducing the flux density in the transformer core, to improve linearity.
2. Discussion of the Prior Art
An electrical isolator circuit (also called an isolation amplifier) is a device for transmitting an analog signal from a source circuit or a sensor (connected to the input side of the isolator) to a destination circuit (connected to the output side of the isolator) without permitting a direct flow of current therebetween. Basically, isolators serve two possible roles in their applications environments. First, an isolator may serve as a safety barrier, preventing destructively large or hazardous voltages, applied to the input side of the isolator either intentionally or accidentally, from reaching whatever equipment or persons may be connected to the output, or destination, side of the isolator. Conversely, since the isolation property is bidirectional, the analog signal source may also be protected from potentials which may arise on the output side of the isolator. This is the situation, for example, when an isolator is used in a medical monitoring application. The human patient may be the source of the analog input signal, connected to the input side of the isolator; a medical instrument, such as an electrocardiogram, may be connected to the output side of the isolator. A second possible use for the isolator is to serve as a differential amplifier to extract a desired normal-mode or differential signal from a (possibly much larger) common-mode signal, allowing measurements relatively immune to large potential differences which might exist between a signal source and a measurement system. An isolator also may be used to transfer a signal between two systems which do not share a common ground.
Various types of isolators exist, including isolators which use optical coupling and isolators which employ radio transmission. However, by far the most common are isolators based upon magnetic (i.e. transformer) coupling.
In a magnetic isolator, the input signal (perhaps after some amplification or conditioning) is supplied to the primary winding of an isolation transformer as modulation on a carrier signal. The output signal from the transformer, appearing on its secondary winding, is demodulated in synchronism with the input carrier, to detect the analog input signal.
A variety of modulation schemes are employed. In one technique, referred to as a half-wave isolator, the input signal is chopped by an electronic switch connected in series with the transformer primary. The switch typically is driven by an oscillator carrier signal to close the series circuit to the transformer primary on alternate half-cycles. A corresponding demodulator switch is employed in series with a transformer secondary winding to recover the DC signal level. The demodulator switch is driven by the same oscillator signal as the modulator switch, to establish synchronism between the two switches.
According to another technique, known as double-balanced or full-wave modulation, positive and negative amplitude modulated pulses are supplied to the primary winding of the transformer, in response to the input signal. This method does not require a rectification smoothing circuit or low-pass filter, and therefore has very wide band transmission characteristics.
In many applications of isolators, the precise characteristics of the analog input signal are of interest. There is therefore a great deal of concern about maintaining linearity between input and output signals and with the minimization of drift. Non-linearity and drift introduce unwanted errors into the transfer characteristics of the isolator.
A number of types of errors are attributable to losses occurring in the transformer core. Several mechanisms contribute to such core loss, but all have the property of being substantially proportional in some way to flux density. By minimizing flux (and, thus, flux density), therefore, these losses may be minimized. The problem, of course, is how to get information transmitted through the transformer without significant flux. The first approach taken by prior art designers, referred to as explicit flux nulling, requires the addition of a supplementary secondary transformer winding and a feedback circuit. The supplementary winding is used to sense the presence of flux in the transformer. The signal from the supplementary winding is amplified at high gain and supplied with opposite phase to another winding, to annihilate the original flux created by the input signal. The feedback required to null the input signal is measured and used as an indication of what the input signal must have been. This method was eventually improved by combining the sensing and feedback windings into one. Additionally, improvements were made in the modulation and demodulation techniques, to minimize the effects of switching transients. Regardless of these improvements, however, flux-nuller circuits tend to be subject to thermal drift--that is, they have characteristics which are sensitive to the temperature coefficients of resistors. In particular, they are sensitive to temperature-induced variations of resistors in the feedback circuits.